A Transmission Point (TP) in conventional MIMO OFDM cellular systems consists of a set of co-located transmit antennas and corresponds to a serving cell for a given Reception Point (RP). Transmission points exploit closed-loop precoding based on Channel State Information (CSI) feedback to enhance the system throughput. A CSI Reference Signal (CSI-RS) is sent from each antenna of the transmission point and used at the receiver to compute the CSI feedback reflecting the channel quality between a set of co-located antennas at the transmitter and a set of co-located antennas at the receiver. Data and reference signals transmitted from different TPs interfere with each other. To minimize interference, the reference signals transmitted from different points are typically placed in orthogonal resource elements where no data can be transmitted. CSI-RS time-frequency patterns of different TPs are signalled to the receiver from the serving TP, see e.g. 3GPP LTE Rel-10 systems.
More advanced systems exploit Coordinated Multi-Point (CoMP) operation to enable coordinated Downlink (DL) transmission and/or Uplink (UL) reception from/at multiple geographically separated points, forming a so called CoMP cooperating set. CoMP was agreed, for instance, as part of the 3GPP LTE Rel-11, both in downlink and in uplink (3GPP TR 36.819, “Technical Specification Group Radio Access Network; Coordinated Multi-Point Operation for LTE Physical Layer Aspects” (Rel-11)).
Joint Processing (JP) is used to make data for a UE available at multiple TPs within the CoMP cooperating set. Joint Transmission (JT) allows simultaneous data transmission from multiple points to a single or multiple UEs in a time-frequency resource. Dynamic point selection (DPS)/muting can enable data transmission from one point in a time-frequency resource. Through Coordinated Scheduling/Beamforming (CS/CB), data for a UE is only available at, and transmitted from, one point in the CoMP cooperating set for a time-frequency resource but UE scheduling/beamforming is coordinated among points in the CoMP cooperating set.
Coordinated multi-point reception implies coordination in UL between geographically separated RPs, such as Joint Reception (JR) of the physical uplink shared channel (PUSCH) transmission at multiple RPs, and/or coordinated scheduling (CS) decisions among points to control interference and improve coverage.
With coordinated multi-point operation, a more advanced receiver may be signalled several sets of CSI-RS patterns by the transmitter. The CoMP cooperating set can consist of macro-cell(s) and low-power Remote Radio Head(s) (RRH) with the same or different cell-ID. Different cell-IDs allow defining orthogonal reference signals that the UE can use to distinguish signals from different TPs. If the same cell-ID is used within the CoMP cooperating set, the UE cannot distinguish the DL signals. This affects, for instance, the propagation loss estimation for power control.
Currently, the UL power control in 3GPP LTE Rel-8/9/10 systems combines an open-loop term and a closed-loop correction term. The UE PUSCH transmit power to a serving cell c in subframe i is given in 3GPP TS 36.213, “Evolved Universal Terrestrial Access (E-UTRA); Physical Layer Procedures”, Release 10 as:
                                          P                          PUSCH              ,              c                                ⁡                      (            i            )                          =                  min          ⁢                      {                                                                                                      P                                              CMAX                        ,                        c                                                                                                                                                                                                          10                          ⁢                                                                                                          ⁢                                                                                    log                              10                                                        ⁡                                                          (                                                                                                M                                                                      PUSCH                                    ,                                    c                                                                                                  ⁡                                                                  (                                  i                                  )                                                                                            )                                                                                                      +                                                                              P                                                          O_PUSCH                              ,                              c                                                                                ⁡                                                      (                            j                            )                                                                          +                                                                                                            α                              c                                                        ⁡                                                          (                              j                              )                                                                                ⁢                                                      PL                            c                                                                          +                                                  Δ                                                      TF                            ,                            c                                                                          +                                                                              f                            c                                                    ⁡                                                      (                            i                            )                                                                                              ,                                                                                  ⁢                                                                                     [                                          dB                      ⁢                                                                                          ⁢                      m                                        ]                                    ,                                                                                        (        1        )            wherePLc=RSTP−RSRP  (2)is the DL propagation loss computed at the UE, RSTP is the DL reference-signal transmit power defined as the linear average over the power contributions of all resource elements that carry Cell-specific ReHference Signals (CRS) within the operating system bandwidth, and RSRP is the reference signal received power as defined in 3GPP TS 36.214, “Evolved Universal Terrestrial Access (E-UTRA); Physical Layer—Measurements”, Release 10; with the higher layer filter configuration provided in 3GPP TS 36.331, “Evolved Universal Terrestrial Access (E-UTRA); Radio Resource Control (RRC) Protocol Specification”, Release 10.
In equation (1), PCMAX,c is the configured UE maximum transmit power as defined in 3GPP TS 36.213, “Evolved Universal Terrestrial Access (E-UTRA); Physical Layer Procedures”, Release 10; MPUSCH,c(i) denotes the instantaneous PUSCH bandwidth measured in number of resource blocks valid for subframe i; P0_PUSCH(j) is a cell-specific parameter broadcasted as part of the cell system information; ΔTF,c reflects the fact that different SINR can be required for different modulation schemes and coding rates used for PUSCH transmission; and αc(j) is a 3-bit cell specific parameter provided by higher layers for partial path loss compensation. The term 10 log10(MPUSCH(i)) reflects the fact that what is fundamentally controlled by the parameter P0-PUSCH(j) is the power per resource block. Ultimately, PPUSCH is tuned to guarantee a minimum received power required to support a specific code and modulation scheme. This UL power control does not immediately apply to CoMP for the difficulty in estimating the UL propagation loss.
The uplink communication paradigm in conventional cellular systems implies a point-to-point communication between a transmission point and a reception point. The uplink propagation loss is univocally defined as the signal attenuation between the transmitter and the receiver. With co-located transmission/reception points in uplink and downlink, the uplink path loss can be estimated by exploiting the DL/UL channel reciprocity, for instance, via the LTE open-loop path loss estimation in equation (2).
The uplink CoMP reception mode, on the other hand, can significantly differ from the uplink of conventional cellular systems. Both the downlink transmission to and the uplink reception from a UE may involve multiple geographically separated points, and the set T of DL TPs may not coincide with the set R of UL RPs as in FIG. 1.
The existence of multiple reception points of the same transmitted signal from a UE in UL CoMP reception mode causes the problem of determining a single basic metric that encompasses the total signal attenuation when a signal is jointly received by multiple points.
The metric, hereafter referred to as virtual propagation loss, shall reflect the amount of power adjustment needed at the UE to maintain a desired total quality of service provided jointly by all multiple reception points. In principle, this metric is a function of all separate propagations losses to each of the reception points, but the open issue is how that function should look like, even on a general level.
Recent work proposed a number of ways to define a propagation loss metric for uplink CoMP reception mode. To unify the description of these proposals, a CoMP cooperating set is considered consisting of K points labelled k=1, . . . , N. Within this set, N≦K points indexed by n=1, . . . , N form a coordinated reception point set R associated with a specific UE (mobile station) Further, let Pk and Lk denote the transmit power (in decibels) of the k-th point in the CoMP cooperating set and the path loss (in decibels) from the UE, respectively, where Pk takes value Pmacro when associated with the downlink transmission from a macro-cell. The value Pmacro is signalled to the UE by higher layers as part of system information.
A simple way to define a path loss for uplink CoMP reception mode is to reuse the LTE rel-8/9/10 open-loop path loss estimation in equation (2) based on downlink measurement of a common CRS reference signal. Assuming that all transmission points share the same cell ID (hence the same CRS reference signal), the RSRP term in equation (2) would account for the superposition of the DL signals from all transmission points, thus yielding
                                          L                          UL              ,              LTE                                =                      RSTP            -                          10              ⁢                                                          ⁢                                                                    log                    10                                    ⁡                                      (                                                                  ∑                                                  k                          =                          1                                                K                                            ⁢                                                                                          ⁢                                              10                                                                              Pk                            -                            Lk                                                    10                                                                                      )                                                  ⁢                                                                  [                dB                ]                                                    ,                            (        3        )            where RSTP is assumed to be the macro-cell transmission power Pmacro.
The metric LUL,LTE, however, can be rather inaccurate since is computed upon a DL transmission from all points in the CoMP cooperating set while the UL reception point set R may consist of fewer points.
Moreover, formally speaking LUL,LTE is not representative of path loss as it admits, at least in theory, negative values. To illustrate this statement, we consider a CoMP cooperating set with a macro cell and K−1 RRHs with RSTP=Pmacro. Let
            P      ~        macro    =                    10                                            P              macro                        -                          L              macro                                10                    ⁢                          ⁢      and      ⁢                          ⁢                        P          ~                n              =                  10                                            P              n                        -                          L              n                                10                    =                        ɛ          n                ⁢                              P            ~                    macro                    denote the received power (in linear-scale) from the macro cell and RRH, where εk is the ratio between the UE's received power from the k-th RRH and the macro cell. Assuming that same ε applies to all RRHs, the term LUL,LTE becomes negative for
  ɛ  >            (                        10                                    L              macro                        10                          -        1            )        /                  (                  K          -          1                )            .      
By analogy with the single transmission/reception point case, another definition of path loss for uplink CoMP reception has been proposed as the difference between the UE's transmit power PUE(dB) and total received power (dB) by all reception points in R:
                              P                      RX            ,            tot                          =                                                            P                UE                            +                              10                ⁢                                                                  ⁢                                                      log                    10                                    ⁡                                      (                                                                  ∑                                                  n                          =                          1                                                N                                            ⁢                                                                                          ⁢                                              10                                                  -                                                                                    L                              n                                                        10                                                                                                                )                                                                        ⇒                          L                              UL                ,                eff                                              =                                                    P                UE                            -                              P                                  RX                  ,                  tot                                                      =                                          -                10                            ⁢                                                          ⁢                                                                    log                    10                                    ⁡                                      (                                                                  ∑                                                  n                          =                          1                                                N                                            ⁢                                                                                          ⁢                                              10                                                                              L                            n                                                    10                                                                                      )                                                  ⁢                                                                  [                dB                ]                                                                        (        4        )            
A power control compensating LUL,eff, referred to as effective path loss, regulates the total received power by all RPs. Notice that LUL,LTE=LUL,eff if and only if the downlink transmission point set T and the uplink reception point set R are the same and all transmission points use the same RSTP.
However, a power control scheme that compensates for the metric LUL,eff would not produce enough power to compensate even the path loss toward the closest RP. To prove this statement, we consider an UL transmission from the UE with power PUE. The (linear-scaled) total received power by all RPs PRX,tot in equation (4) is larger than (or equal to) the power received by the RP with the smallest propagation loss, i.e.
                                                        ∑                              n                =                1                            N                        ⁢                                                  ⁢                          10                                                                    P                    UE                                    -                                      L                    n                                                  10                                              ≥                                    max                                                n                  =                  1                                ,                …                ,                N                                      ⁢                                          {                                  10                                                                                    P                        UE                                            -                                              L                        n                                                              10                                                  }                            ⁢                                                          ⁢                              (                                  linear                  ⁢                                      -                                    ⁢                  scale                                )                                                    ,                                  ⁢        hence                            (        5        )                                          L                      UL            ,            eff                          ≤                              min                                          n                =                1                            ,              …              ,              N                                ⁢                                    L              N                        .                                                  ⁢                          [              dB              ]                        .                                              (        6        )            
Therefore, the transmit powers PUE,eff and PUE,min that would be produced to compensate the path loss LUL,eff and
            L              UL        ,        min              =                  min                              n            =            1                    ,          …          ,          N                    ⁢              L        N              ,respectively, are related asPUE,eff≦PUUE,min. [dB]  (7).
To gain more insight into this aspect, we quantify the difference ΔL=LUL,min−LUL,eff (in decibels) for a heterogeneous network deployment with a macro cell and four RRHs. The macro cell radius is 500 m, and the RRHs are placed by the cell-edge, with 200 m horizontal-spacing and 100 m vertical-spacing as shown in FIG. 2. ΔL is computed for each UE position by moving the UE along a grid with step 10 m.
In general, ΔL depends on the UE position (which affects the minimum propagation loss), as well as on how many and which RPs are within the set R. It follows straightforwardly from equation (6) that ΔL≧0 for any UE position. FIGS. 2(a)-2(b) illustrate the contour magnitude of ΔL=LUL,min−LUL,eff when the reception point set R consists of the two best RPs for each UE position, i.e. LUL,eff is computed using the two RPs with the smallest propagation loss, and when R consists of all points, i.e. macro site and four RRHs, respectively. FIG. 3 is the Cumulative Distribution Function (CDF) of the difference ΔL (dB) between the minimum propagation loss LUL,min for each UE position and the path loss to other RPs, taken in increasing order.
The analysis in FIG. 2 indicates that the transmit power produced to compensate for the metric LUL,eff would yield a Signal-to-Noise Ratio (SNR) at the closest RP several dBs smaller than the corresponding SNR if the power control compensated for LUL,min. FIG. 3 further suggests that the SNR at the closest and the second closest RPs can differ up to 35 dB, and can be larger for other RPs. Therefore, the propagation loss metric LUL,eff may not be the best candidate for power control with multiple reception points.
Another metric defining the propagation loss in a system with multiple reception points includes a linear average of the individual propagation losses Ln (expressed in linear scale) with per-point path loss weights wn as follows
                                          L                          UL              ,                              lin                -                avg                                              =                      10            ⁢                                                  ⁢                                          log                10                            ⁡                              (                                                      ∑                                          n                      =                      1                                        N                                    ⁢                                                                          ⁢                                                            w                      n                                        ⁢                                          10                                                                        L                          n                                                10                                                                                            )                                                    ,                            (        8        )            or a nonlinear average of the individual propagation losses Ln expressed in linear scale
                              L                      UL            ,            nonlin                          =                  10          ⁢                                          ⁢                                                    log                10                            ⁡                              (                                  10                                      10                    /                                          (                                                                        ∑                                                      n                            =                            1                                                    N                                                ⁢                                                                                                  ⁢                                                  1                          Ln                                                                    )                                                                      )                                      .                                              (        9        )            
In alternative, it was proposed to adjust the UE transmit power based on the propagation loss to the closest/furthest RP.